Mapping of the Ocean Floor

The ocean floor is not a flat, featureless plain. Detailed research into its configuration has revealed a landscape as varied as any on land — full of relief, with mountains, plains, and deep trenches. This understanding came only after sustained effort, because the ocean floor is hidden beneath thousands of metres of water and cannot be observed directly.

The turning point was the period after World War II, when expeditions specifically designed to map the oceanic floor were undertaken. These expeditions produced the first detailed picture of ocean relief. They revealed two striking features: submerged mountain ranges and deep trenches, with the trenches located mostly closer to the continent margins. The mid-oceanic ridges — the long mountain chains running through the oceans — were found to be the most active zones in terms of volcanic eruptions.

The most revealing evidence came from dating the rocks of the oceanic crust. These rocks turned out to be much younger than the rocks of the continental areas. Furthermore, when rocks on either side of the crest of an oceanic ridge were compared, those located at equal distances from the crest showed remarkable similarities — both in their constituents and in their age. This symmetry on both sides of the ridge crest became a crucial clue for understanding how the ocean floor forms and spreads, a point that underpins the theory of seafloor spreading discussed later in the chapter.